Presently, there are many types of vaccines used to immunize individuals against disease. Although those available are generally safe and, at least to some extent, effective in inducing an immune response, all have limitations. In addition, for some diseases, there is no effective vaccine. The development of alternatives to presently-available vaccines and of new vaccines to protect against diseases for which no vaccine is currently available would be an important step in reducing the morbidity and mortality caused by many diseases.